Syntactic unary operator should not look across newline

[yuyichao]

As brought up in #11338 (comment) .

IMHO, if a expression/statement is complete at the end of line, it should not look across the new line to continue.

julia> function parseall(str)
           pos = start(str)
           exs = []
           while !done(str, pos)
               ex, pos = parse(str, pos)
               push!(exs, ex)
           end
           if length(exs) == 0
               throw(ParseError("end of input"))
           elseif length(exs) == 1
               return exs[1]
           else
               return Expr(:block, exs...)
           end
       end
parseall (generic function with 1 method)                                         

julia> parseall("&")
:&                                                                                

julia> parseall("&\na")
:(&a)                                                                             

julia> parseall("&\n")
:($(Expr(:incomplete, "incomplete: premature end of input")))                    

(parseall taken from #11495)